Heavy duty diesel engines require crankcase lubricant oils that contain additives which stabilize the oil against oxidation and which are non-corrosive to silver members of the engine. Oxidative deterioration is undesirable because it is accompanied by the formation of gum, sludge and acids which cause metal corrosion, chemical breakdown of the lubricant, and an increase in the viscosity of the lubricant. It is important that silver-containing connecting rod bearings and lead surfacing thereon are not attacked either by additives in the oil or by the oxidative, neutralized or decomposition products thereof during extended use.
High alkalinity in the lubricating oil is required to neutralize acidic combustion products and to extend the time intervals between oil changes. Also, the alkalinity life of the oil should be long in order to reduce the need to add alkalinity imparting additives before a complete change of the oil, and to reduce the frequency of oil changes. As is generally well known, most of the conventional, inexpensive sources of alkalinity, e.g., overbased phenate or sulfonate, are aggressive toward silver. This results in excessive wear of the silver-containing parts of the engine. The harshness of these overbased additives to silver has been attributed to the presence of carbonate, e.g., calcium carbonate, magnesium carbonate and the like.
Carbonate overbased additives can be tolerated in the lubricating oil by treating the oil with silver passivating or silver lubricity agents, e.g., chlorinated waxes and glycerol monooleate or pentaerythritol monooleate, as described in U.S. Pat. No. 4,734,211 to Kennedy. However, such silver lubricity agents can impart undesirable effects on the lubricating oil's performance. One such detrimental effect is oxidative thickening. Silver wear or lubricity agents are employed in spite of their undesirable side effects and cost because they allow the use of inexpensive alkalinity sources.
U.S. Pat. No. 3,567,637 to Sabol generally discloses the use of alkaline earth long chain alkenyl succinates as detergent additives for diesel oils. U.S. Pat. No. 4,710,308 to Stauffer generally discloses the use of overbased sulfurized phenates as antioxidants and dispersants. However, the silver-mildness of alkaline earth overbased polyalkenyl-substituted succinates when used in heavy duty diesel engines having silver parts in certain amounts is neither disclosed nor suggested.
U.S. Pat. No. 4,171,269 to Sung et al. teaches and claims a railway diesel engine lubricating oil composition having a TBN of at least 10 wherein the composition comprises a sulfurized normal or highly overbased calcium alkylphenolate detergent-inhibitor, a highly overbased alkaline earth metal hydrocarbyl sulfonate, a sulfurized naphthenic lubricating oil incorporating from about 1 percent to about 6 percent by weight of elementary sulfur and from 0.05 weight percent to 5 weight percent of a chloroparaffin wherein there is contained in combined form from 40 percent to 60 percent by weight of chlorine. The sulfurized naphthenic lubricating oil additive preferably contains a sulfurized lard oil formed essentially of triglycerides of C.sub.12 to C.sub.20 fatty acids and containing preferably triglycerides of myristic, palmitic and stearic, oleic and linoleic in concentrations of 1, 26, 11.5, 58 and 3.5 weight percent, respectively. The amount of chloroparaffin present will correlate generally with the amount of calcium sulfonate and be within the range of from 0.05 weight percent to 5 weight percent of the total lubricant composition. Silver wear properties were poor for formulations not containing both the chloroparaffin additive and the sulfurized naphthenic oil.
U.S. Pat. No. 4,278,553 to Sung et al. teaches and claims a railway diesel engine lubricant containing a silver corrosion inhibitor comprising a benzotriazole compound present in concentrations from about 0.5 to 2.0 weight percent. Examples of silver corrosion inhibitors include benzotriazole derivatives of N-alkyl-3-propanediamines.
U.S. Pat. No. 4,169,799 to Sung et al. discloses a combination of components consisting of an overbased alkaline earth metal containing alkyl phenolate sufficient to impart a TBN of at least 10 alkylphenol and a chlorinated sulfurized alkylphenol in a mineral oil base stock. The chlorinated alkylphenol is present in an amount of from 0.25 to 20 weight percent.
U.S. Pat. No. 4,285,823 to Sung et al. discloses a silver corrosion inhibitor for railway diesel engines lubricants comprising an N-alkylaminomethyl-5-amino-1H-tetrazole. The diesel lubricant contains the additive in an amount of from 0.5 to 2.0 weight percent.
Use of chlorinated hydrocarbons as silver wear inhibitors in railway diesel engine oils is also taught in U.S. Pat. Nos. 4,320,016 to Zoleski et al.; 4,428,850 to Zoleski et al. and 4,464,276 to Sung et al.
U.S. Pat. No. 4,734,211 to Kennedy discloses the use of lubricating oil compositions to provide silver wear inhibition and alkalinity reserve. The composition includes an overbased alkaline earth metal alkylphenolate, an alkaline earth metal alkylsulfonate, an overbased alkaline earth metal phenolate, a chlorinated hydrocarbon and a polyhydroxy compound such as glycerol monooleate.
In the above-mentioned patents, lubricant compositions useful as railway diesel engine lubricants are those containing substantially normal and/or highly overbased alkaline earth metal sulfurized alkylphenolate and highly overbased alkaline earth metal sulfonate additives generally having a TBN of at least 10 (Kennedy has a TBN of at least 5) and thus capable of preventing corrosion by oil-soluble acids formed by oxidative deterioration under normal engine use. If a sulfurized naphthenic oil-containing composition having a sufficient sulfur content is present with the foregoing overbased additives, the corrosion of the silver-plated bearings by the overbased alkaline earth metal alkylphenolate is overcome but not the similarly destructive properties of the alkaline earth metal sulfonate. Nevertheless, the incorporation of an alkaline earth metal sulfonate in these lubricant oils is desirable because of the improved engine performance provided over an extended period of time.
In addition, it is generally well known that zinc containing wear inhibitors such as the zinc dihydrocarbyldithiophosphate wear inhibitors, while commonly used in passenger car crankcase lubricants, cannot be tolerated in diesel engines containing silver parts and bearings because the zinc dithiophosphates are harmful to silver. Diesel engine lubricants, in particular railway diesel lubricants, therefor do not contain zinc dithiophosphate wear inhibitors. The antagonism of zinc containing wear inhibitors toward silver parts is mentioned in U.S. Pat. No. 4,428,850, column 1, lines 63-68.
As already noted, chlorinated hydrocarbons have been incorporated into railway diesel engine lubricant compositions to provide silver protection properties to the lubricant compositions. However, from an environmental standpoint it is generally preferred to reduce or eliminate altogether the use of chlorinated compounds in railway diesel engine lubricants.
An object of the present invention is therefore to provide a lubricating composition and method suitable for lubricating diesel engines having silver parts, such composition incorporating a species of overbased alkaline earth compound which is sufficiently mild toward the silver parts of the engine whereby silver lubricity agents, in particular those containing chlorine, may be avoided or substantially reduced. Other objects appear hereinafter and will become evident to the person of ordinary skill in the art.